1. Field of the Invention
The invention relates to the field of near field optics and more particularly to its use in heat assisted magnetic recording.
2. Description of the Related Art
Heat assisted magnetic recording (HAMR) involves heating a spot on the disk surface to reduce its coercivity sufficiently so that it can be magnetically recorded. The advantage of this technique is that the coercivity of the media at ambient can be significantly increased, thereby improving thermal stability of the recorded data even for very small bit cells. One of the difficulties with the technique is finding a method to heat just the small area of media which is to be recorded. Heating with laser light, as is done in magneto-optic recording, is the most promising approach, but the difficulty with this is that at the current storage densities contemplated for HAMR, the spot to be heated is ˜25 nm in diameter, which is fifty times smaller than the wavelength of useful semiconductor lasers. The so-called diffraction limit in optics is the smallest dimension to which a light beam can be focused. The diffraction limit in three dimensions is given by the equation
                    d        =                              0.6            ⁢            λ                                n            ⁢                                                  ⁢            sin            ⁢                                                  ⁢            θ                                              (        1        )            where d is the spot diameter, λ is the wavelength of the light in free space, n is the refractive index of the lens, and θ is the maximum angle of focused light rays from the central axis of the lens. The factor 1/n is the wavelength of the light within the lens. The spot diameter is directly proportional to the wavelength of the light within the lens. The minimum focused spot diameter in the classical diffraction limit is ˜λ/2, which is much too large to be useful for HAMR.
When light is incident upon a small circular aperture, it is well-known in classical optics that the amount of power transmitted through the aperture scales as the ratio of the aperture to the wavelength raised to the fourth power [H. A. Bethe, “Theory of Diffraction by Small Holes” Phys. Rev. 66 (1944) 163-182]. In other words, the amount of light which can be transmitted through an aperture with a ˜25 nm diameter at a wavelength of 500 nm is ˜6×106 of the amount that would be expected for the size of the hole. This throughput is orders of magnitude too small to be practical for HAMR.
Therefore, there is a need to focus or confine energy from a light source having a wavelength on the order of 500 nm or greater into a spot whose diameter is on the order of 25 nm with high transmission efficiency. The relevant art provides no solution.